Re: Starlink routing

[Tom Beecher <beecher () beecher cc>]

Appreciate that. Definitely becoming clear to me that a lot of my knowledge
here was rusty. Lots of papers on this specifically (Doppler effects on
optical ISL) that I need to call in some favors to get access to.

Thanks!

On Mon, Jan 23, 2023 at 1:08 PM Thomas Bellman <bellman () nsc liu se> wrote:

> On 2023-01-23 17:27, Tom Beecher wrote:
>
> > What I didn't think was adequately solved was what Starlink shows in
> > marketing snippets, that is birds in completely different orbital
> > inclinations (sometimes close to 90 degrees off) shooting messages to
> each
> > other. Last I had read the dopplar effects there were so much larger due
> to
> > relative speed deltas it just couldn't currently be done. If there is
> more
> > out there on that solution, be glad to read up on what info anyone may
> have
> > on that if they can share.
>
> Worst case would be if the satellites are moving directly towards or
> directly away from each other.  Each satellite will be moving at a
> speed of slighly under 8 km/s, and they will thus approach or depart
> from each other with a relative speed of somewhat less than 16 km/s.
>
> I get that for 1310 nm light, the doppler shift would be just under
> 0.07 nm, or 12.2 GHz:
>
>     l0 = 1310 nm
>     f0 = c / l0
>     f = f0 / sqrt((1 + 16 km/s / c) / (1 - 16 km/s / c))
>     l = c / f ≈ 1310.0699 nm
>     f0 - f ≈ 12.2 GHz
>
> In the ITU C band, I get the doppler shift to be about 10.5 GHz (at
> channel 72, 197200 GHz or 1520.25 nm).
>
> (Formula from https://en.wikipedia.org/wiki/Relativistic_Doppler_effect
> first entry in the table under "Summary of major results".)
>
> These shifts are noticably less than typical grid widths used for
> DWDM (±50 GHz for the standard spacing), so it seems unlikely to me
> that the doppler shift would be a problem.
>
>
>         /Bellman